GB2101401A

GB2101401A - Glass encapsulated semiconductor device

Info

Publication number: GB2101401A
Application number: GB08216382A
Authority: GB
Inventors: Bulent Erturk Yoldas
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1978-04-18
Filing date: 1979-04-12
Publication date: 1983-01-12
Also published as: FR2423866A1; JPS54140872A; BR7902283A; CA1123522A; BE875606A; GB2019089B; US4168960A; IN150707B; DE2915631A1; GB2019089A; FR2423866B1; GB2101401B

Description

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GB 2 101 401 A 1

SPECIFICATION

Glass encapsulated semiconductor device

The invention relates generally to semiconductor devices and more specifically to 5 glass encapsulated semiconductor devices. The subject of the present application has been divided from co-pending application No. 7913125.

Prior art semiconductor diodes using glass 10 fused directly to the semiconductor portion of the diode as the sole means of protecting the PN junction from the environment have been limited to relatively low current diodes. An example of such a diode is type UT4005 manufactured and 1 5 sold by the Unitrode Corporation. It is also known in the prior art to encapsulate semiconductor devices in thermosetting resinous insulating material. Examples of such hermetically sealed diodes using resinous material are disclosed in 20 Patent Nos. 3,475,662, 3,476,987 and 3,476,988 as well as 3,486,084. Thin glass protective layers are also available in the prior art to passivate large prior art semiconductor devices. Glass forming these layers was typically 25 applied to the body of semiconductor as a slurry and the device and the powdered glass were heated to fuse the glass and for protective glass layer. Glass layers formed using this technique were limited to thicknesses in the order of 20 to 30 30 microns. These thin layers are not sufficient to provide complete environmental protection for PN junctions and other circuit elements within the body of semiconductor material.

The invention consists in a semi-conductor 35 device comprising a semiconductor device body having opposed surfaces with an edge therebetween; and a glass member disposed about the body and having an inner surface fused to said edge, said glass member having a first 40 region of a lead borosilicate glass fused to said edge and a second region of a zinc borosilicate glass disposed about said first region.

Exemplary embodiments of the invention are described in the following drawings:

45 Figure 1 is a drawing illustrating the preferred embodiment of the semiconductor device i.e. a diode, shown in cross-section.

Figure 2 is a drawing of the body of semiconductor material utilized by the diode 50 illustrated in Figure 1.

Figure 3 is a drawing of the top electrode of the diode illustrated in Figure 1.

Figure 4 is a drawing of the bottom electrode of the diode illustrated in Figure 1.

55 Figure 5 is a drawing of the ring-like glass member utilized by the diode illustrated in Figure 1.

Figure 6 is a drawing illustrating the prefabricated glass rings used in constructing the 60 diode illustrated in Figure 1.

Figure 7 is a diagram illustrating the jig used to support the fusion and prefabricated glass rings in the fusion furnace.

Figure 8 is a time temperature profile of the

65 fusion furnace.

Figure 9 is a drawing illustrating the relationship between the leakage current of the diode and the partial pressure of the water vapor in the fusion furnace.

70 Figure 10 is an alternate embodiment of the jig utilized to support the fusion and glass preforms in the furnace.

Figure 11 is a diagram illustrating the contraction characteristics of glass.

75 Referring to Figure 1, diode 20 is shown. Diode 20 utilizes a fusion consisting of a body of semiconductor material 22 (preferably silicon, separately illustrated in Figure 2) and first and second electrodes 24 and 26 (separately 80 illustrated in Figure 3 and 4). Electrodes 24 and 26 are preferably a refractory metal such as, for example, molybdenum, tungsten, tantalum base alloys and mixtures thereof.

The body of semiconductor material 22 85 includes a PN junction 27 at the interface of P conductivity type region 28 and N conductivity type region 30. The P and N conductivity type regions, 28 and 30 respectively, extend from the PN junction 27 to upper and lower surfaces, 32 90 and 34 respectively, of the body of semiconductor material 22. Electrode 24 is preferably cup-shaped and includes a lower surface 33 which is affixed to the upper surface 32 of the body of semiconductor material 22. Similarly, electrode 95 26 includes an upper surface 38 which is affixed to the lower surface 34 of the body or semiconductor material 22.

The fusion consisting of the body of semiconductor material 22 and electrodes 24 and 100 26 is encircled by a preferably coaxially positioned annularly shaped or ring-like electrically insulating glass member 40, which is separately illustrated in Figure 5. Inner surface 42 of ring-like glass member 40 is fused to edge 105 portion 44 of the body of semiconductor material 22 as well as to outer surfaces, 48 and 50, of the electrodes, 24 and 26. Ring-like glass member 40 includes two regions, 40a and 40b. Region 40a is preferably a lead-aluminum-borosilicate glass 110 such as IP745 sold commercially by Innotech. Region 45b is preferably a zinc-borosilicate glass such as IP660 also sold commercially by Innotech. These glasses are described in detail hereinafter.

115 A fused junction is formed along the inner surface 42 of annular shaped glass member 40 and the outer edge portion 44 of the body of semiconductor material 22. This fused junction provides a hermetic seal or encapsulation 120 protecting the PN junction 27. However,

additional protection is provided by the fused junction between the inner surface 42 of a ring-like glass member 40 and the outer surfaces, 48 and 50, of the electrodes, 24 and 26.

125 Bottom electrode 26 extends beyond the lower surface of the annular shaped glass member 40. This permits contact to be made with the bottom electrode 26 through a flat surface without

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interference by annularly shaped glass member 40.

Embodiments of the diode 20 actually constructed have a voltage rating of 600 volts, a 5 current rating of 1 50 amps.

Diodes were constructed in which the glass member 40 was a lead borosilicate glass such as IP745 sold by Innotech Corp. Also, diodes in which the glass member 40 included the first 10 region 40a of lead borosilicate glass and the second region 40b of zinc borosilicate glass, such as IP660 also sold by Innotech Corp. have been constructed. The combination of the lead borosilicate and the zinc borosilicate glasses have 1 5 been found to yield superior results. The lead borosilicate glass has a composition by weight of 36±4% Si02, 1 5±3% B203, 45±3% PbO and 3±1% Al203. The zinc borosilicate glass has a composition by weight of 55±5% ZnO, 31 ±4% 20 B203, 8±2% Si02, 4.5±1% CeO and approximately 1.0% Al203.

The first step in constructing the diode 20 is to affix the bottom electrode 26 and the top electrode 24 to the body of semiconductor 25 material 22 to form the diode body sometimes also called a fusion. In the preferred embodiment bottom electrode 26 is affixed to the body of semiconductor material 22 by silver soldering the top surface 38 of electrode 26 to the bottom 30 surface 34 of the body of semiconductor material 22. Silver solders are available to permit this process to be carried out at a temperature ranging from 800°C to 900°C. The silver solder may be an alloy of lead, tin and silver. These solders are 35 commercially available.

The top electrode 24 is affixed to the top surface 32 of the body of semiconductor material 22 by soldering or brazing the bottom surface 33 of electrode 24 to the top surface 32 of the body 40 of semiconductor material 22 using aluminum. Suitable prior art processes are available for performing this operation at a temperature ranging from 500° to 550°C.

In general, electrodes 24 and 26 may be 45 affixed to the body of semiconductor material 22 using any suitable process known to those skilled in the art.

After the electrodes, 24 and 26, have been affixed to the body of semiconductor material 22 50 the edge 44 of the body of semiconductor material 22 is beveled to complete the diode body. The beveling is preferably carried out by sandblasting followed by a chemical polishing and etching in a mixture consisting of hydrofluoric, 55 nitric and acetic acids. This polishing technique is well-known in the semiconductor industry and can be performed using commercially available etchants and equipment.

Diode 20 is constructed from the diode body 60 described above and first and second prefabricated glass rings, 54 and 56 (separately illustrated in Figure 6).Two preformed glass rings are used because this permits the glass overlying the PN junction 27 (Fig. 1) to be selected to 65 optimize the protection of the PN junction 27 and the remainder of the glass to be selected based on its electrical insulation, thermal and mechanical properties.

The first, step in constructing the diode 20 is to clean the diode body and the prefabricated glass rings, 54 and 56, using the following procedure:

A. Boil all the components in reagent grade trichloroethylene;

B. Rinse all the components twice (one minute each time) in reagent grade trichloroethylene;

C. Rinse all the components ultrasonically twice (one minute each time) in reagent grade acetone;and

D. Dry in room air on filter paper.

Following cleaning as described above, all the components are assembled in a jig as illustrated in Figure 9. The jig utilizes a graphite base member 58 having a recess 60 therein. The recess 60 is circular and has a diameter slightly larger than the diameter of the bottom electrode 26. This permits the diode body to be assembled in the jig such that the bottom electrode 26 is in the recess 60 in the base member 58.

The second prefabricated glass ring 56 is placed in concentric relationship with the top electrode 24 of the diode body. The first prefabricated glass ring 54 is then positioned concentric with the fusion and the second prefabricated glass ring member 56.

The base member 58 of the fixture includes two guide pins, 64 and 66. Each of the guide pins, 64 and 66, includes lower, middle and upper portions 66a, 66b, 66c and 64a, 64b and 64c. Portions 64a and 64c of guide pin 64 are smaller than portion 64b. Similarly, portions 66a and 66c are smaller than portion 64b. Each of the guide pins are positioned in a hole in the base member 58 such that the center portions 64b and 66b, are supported on the upper surface 58a of base member 58.

A top plate 68 is then positioned as shown in Figure 9. The fusion and the prefabricated glass rings, 54 and 56, as assembled in the jig and illustrated in Figure 7, are then placed in a fusion furnace having an initial temperature in the range of 350°C and heated in a controlled atmosphere to fuse the prefabricated glass rings 54 and 56 to produce the ring-like glass member 40, consisting of two regions 40a and 40b, as illustrated in Figure 1.

The preferred control atmosphere mentioned above consists of a mixture of nitrogen and water vapor having a total pressure of one atmosphere with the partial pressure of the water vapor being greater than 10~4 atmospheres and preferably in the range of 10~4 to 10-2, 10-3-5 to 10~1 or 10-3 to 10~2 atmospheres. The required water vapor is achieved by mixing approximately 2 parts of dry nitrogen with one part of wet nitrogen and flowing the mixture through the furnace. Dry nitrogen is passed through 2-1/2 cm. of deionized water in a bubbler to form the required wet nitrogen. After the required atmosphere has been established, the temperature in the furnace is increased and decreased in accordance with

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the time temperature chart illustrated in Figure 8.

As can be seen from Figure 8, the temperature of the furnace is initially in the range of 350°C. The temperature is increased to a temperature in 5 the range of 700° to 720°C in a time interval of 70 approximately 25 minutes. This temperature is maintained for a period of approximately 20 minutes. The prefabricated glass rings, 54 and 56, become soft and begin to flow at a temperature 10 below 700°C. Wettability of glass for silicon and 75 pressure due to top plate 68 causes the soft glass to flow evenly along the edges of semiconductor body 22 and the outer edges 48 and 50 of electrodes 24 and 26. The larger portions 64b 15 and 66b of guide pins 64 and 66 limit the 80

downward motion of the top plate 68 when the prefabricated glass rings, 54 and 56, soften and the glass flows. The height of the larger portions 64b and 66b of guide pins 64 and 66 determines 20 dimension "C" Figure 1. The surface tension of 85 the soft glass causes the outer edge of the glass to form the circular shape as illustrated in Figure 1. Small substantially flat areas may also be formed due to the interfaces of the base member 25 58 and the top plate 68 with the molten glass. 90 Additionally, it should be noted that the top electrode 24 is affixed to the body of semiconductor material 22 by brazing with aluminum. The silicon-aluminum alloy produced 30 by this brazing melts below 600°C. However, 95

using the disclosed process the top electrode 24 remains attached and the melting of the silicon-aluminum alloy does not degrade the PN junction 27.

35 Next, the furnace is cooled from approximately 100 720°C to a temperature in the region of 525°C (i.e. 520° to 530°C) in about 10 minutes. The furnace temperature is maintained in this range for approximately 10 minutes followed by a 40 reduction to a temperature in the region of 480°C

(i.e. 475° to 485°C) in about 15 minutes. This 105 temperature is maintained for approximately 30 minutes followed by a reduction of the furnace temperature to room temperature at a rate of 45 approximately 10°C per minute. This thermal cycle fuses the prefabricated glass rings, 54 and 56, to form ring-like glass member 40 and prevents the formation of possible harmful 110

stresses therein.

50 In selecting the lead borosilicate glass and zinc borosilicate glass for the prefabricated glass rings 54 and 56, respectively, it is important that the thermal expansion coefficients for the glass be matched to or greater than the temperature 11 5

55 expansion coefficients for the semi-conductor body 22. It should also be noted that the expansion characteristics of the glass with temperature are different from the contraction characteristics when the glass is cooled. All of 120 60 these characteristics must be considered in selecting the glass and the temperature cycle for the fusion furnace.

Glasses suitable for use in this invention should have a temperature expansion coefficient in the 125 65 range of 4.0 to 6.0x 10~6 cm/cm°C and the glass for the first prefabricated glass ring 54 which passivates the PN junction 27 should be substantially free of alkali ions. It is also preferable, although not required, that the thermal expansion coefficient of the second prefabricated glass ring 56 be slightly larger than the thermal expansion coefficient of the first prefabricated glass ring 54. In order to maintain the glass adjacent the PN junction 27 (Figure 1) in compression. In addition;

(1) the glasses must have structural stability e.g., must not devitrify or go through detrimental phase separation during the fusion process;

(2) the glass must have good chemical resistance to the environment and humidity;

(3) the glass must have thermal expansion characteristics compatible with those of the fusion;

(4) the glass adjacent the silicon semiconductor body 22 must wet and adhere thereto;

(5) the glass must have a viscosity low enough to flow;

(6) the glass must not chemically attack the surfaces of the semiconductor or the electrodes in a detrimental way;

(7) the thermal characteristics of the glass must be such that stresses can be relieved at temperatures within the limitations of the diode;

(8) the glass must have a fusion temperature below the degradation temperature of the device;

(9) the finished device must be resilient against thermal shock, thermal cycling and have good mechanical strength.

Lead borosilicate glasses having a composition by weight of:

Constituent

Percent

Si02

32—40%

B203

12—23%

PbO

42—48%

ai2o3

2—6%

have been found to be suitable for the first prefabricated glass ring 54.

In particular, a glass having a composition of

Constituent Si02 B203 PbO Al203

% by Weight 3 6 ±4% 15±3% 45±3% 3±1%

have been found to be particularly satisfactory. This glass is sold commercially by Innotech under type No. IP-745.

The characteristics of the second prefabricated glass ring 56 are not as strenuous as those for the first prefabricated glass ring 54 in that the glass comprising the second.prefabricated glass ring 56 can have more alkali ions present. It is also preferable that the second prefabricated glass ring 56 have a temperature expansion coefficient slightly larger than the first prefabricated glass ring 54 so that the glass adjacent the PN junction

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27 (Fig. 1) is maintained in compression. A zinc borosilicate glass particularly suitable for the second prefabricated glass ring 56 has a composition by weight of:

5

Constituent

% by Weight

ZnO

55+5%

b2o3

31+4%

Si02

8+2%

CeO

4.5±1 %

10

ai2o3

1.0%

(approximately)

This glass is sold commercially by Innotech under the type No. IP660.

The prefabricated glass rings, 54 and 56, used 1 5 to construct the diode 20 have the following dimensions:

Dimension

Identification Dimension

Character in inches

20 K (Fig. 8) 0.630

L (Fig. 8) 0.100

M (Fig. 8) 0.080

N (Fig. 8) 0.500

0 (Fig. 8) 0.015

25 In the preferred embodiment descussed above, the two prefabricated glass rings, 54 and 56, are fused to form the composite ring-like glass member 42 illustrated in Figure 1. The ring-like member 40 has two regions 40a and 40b. The 30 first region 40a is composed essentially of the IP745 glass while the second region 40b is comprised of the IP660 glass. It has been found that the most successful combinations is to have the first prefabricated glass ring 54 of IP745 glass 35 while the second prefabricated glass ring 56 is IP660 type glass. Glass type IP745 is composed by weight of 36+4% Si02, 1 5+3% B203, 45±3% PbO and 3±1% Al203. Glass type IP660 is composed by weight of 55±5% ZnO, 31 ±4% 40 B203, 8±2% Si02, 4.5±Ce0 and approximately 1.0% Al203.

Superior results have also been achieved by utilizing prefabricated glass rings, 54 and 56, which are cut from stress relieved glass tubing. 45 These superior results are believed to be related to the fact that prefabricated glass rings of this type have smoother interior surfaces and consistent prior thermal histories, i.e., they are all pulled from a melt.

50 Figure 9 is a curve illustrating the relationship between the partial pressure of the water vapor in the fusion furnace and the leakage current of finished diodes. For example, at a water vapor partial pressure of 10-3-5 atmospheres the 55 leakage current is in the range of 60 milliamps. An order of magnitude decrease in the leakage current is achieved by increasing the partial pressure of the water vapor to 10~25 atmospheres. As previously noted, the preferred 60 water vapor pressure has been found to be in the range of 10~2 to 10~3 atmospheres with a total furnace pressure of one atmosphere.

If for some reason, it is desirable to operate the fusion furnace at a pressure other than one atmosphere, the partial pressures of the nitrogen and water vapor should be adjusted to maintain the proper ratio between the water vapor and the nitrogen. It is also possible to use other inert gases, argon for example, rather than nitrogen.

The embodiment illustrated in Figure 1, can be constructed using an alternate jig as illustrated in Figure 7 or a jig as illustrated in Figure 10.

The jig in Fig. 10 includes a graphite base member 70. In assembling the components of the diode of Patent No. 2017403 in the jig the fusion is positioned such that lower electrode 26 is in a first recess 72 in graphite base member 70. A second recess 74 is concentric with the first recess 72. The prefabricated glass rings 54 and 56 are positioned concentric with the fusion. The prefabricated glass ring 54 has an inner diameter larger than the outer diameter of the diode body such that the lower edge of this preform rests in recess 74. A metallic ring 52 is then positioned concentric with the prefabricated glass ring.

A graphite support cylinder 76 having a recess 78 along its inner wall is then placed over the metal ring 52. The larger recess 74 in the base member 70 and the recess 78 in the inner wall of support cylinder 76 are such that the upper surface of the metal ring 52 is in contact with both the base member 70 and the support cylinder 76.

A graphite pressure cylinder 80 having an outer diameter slightly smaller than the inner diameter than the support cylinder 76 and an inner diameter slightly larger than the outer diameter of the top electrode 24 is then positioned to overlie the prefabricated glass ring 54. A weight 82 is placed on pressure cylinder 80 to complete the assembly of the components in the jig. The combination of the pressure cylinder 80 and the weight 82 is in the range of 20—110 grams. The jig may also support a plurality of diodes, however, each diode should be supported as discussed above.

The components of the diode as assembled and illustrated in Figure 10 are then placed in the fusion furnace and subjected to the controlled atmosphere and temperature cycle previously described with respect to diode 2 illustrated in Figure 1. This causes the prefabricated glass rings 54 and 56 to become soft and fuse to form a ringlike glass member.

An important factor in selecting the time temperature profile for the furnace to assure success of the process discussed above is a careful examination of the contraction characteristics of the glass as well as the temperature characteristics of the materials comprising the diode body.

The thermal expansion of glass is largely determined by the nature of the vitreous network. Unlike the crystalline state encountered in metals and other materials, the vitreous state is not fixed and can vary continuously depending on prior

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heat treatment. In the case of borosilicate glasses, it may even depend on the previous melting history. For example, a rapidly cooled glass has a higher specific volume than the same 5 composition cooled more slowly. Actually, there is no single path that glass follows during contraction. Figure 11 demonstrates that IP-745 glass used in encapsulation the diodes 20 (Fig. 1) can be cooled from 515°C in various ways to give 10 an entirely different contraction path. Glass type IP745 is a lead borosilicate glass having a composition by weight of 36+4% Si02, 15%+3% B203, 45±3% PbO and 3±1 % Al203. The variations in the contracting paths, depending on 15 the way the glass is cooled, are shown as a shaded area in Figure 11. If the glass is stabilized for sufficient time, e.g., 20 minutes, at its deformation temperature of approximately 515°C, it will shrink along a path defined by the 20 upper boundary of the shaded area illustrated in Fig. 11. However, if the same glass is heated close to its deformation point and cooled rapidly, an entirely different vitreous state will be formed and high shrinkage occurs along a path defined by 25 the lower boundary or the shaded area illustrated in Fig. 11. The behavior of glass on cooling is generally considerably different than that on heating and depends to a great extent on the method of annealing and thermal history. Upon 30 cooling, glass will not follow the heating curve and normally will not shrink to its original volume.

The silicon as well as various refractory metals and alloys thereof which might be considered for component parts of the diode have relatively well 35 behaved temperature characteristics. That is, they tend to expand and contract along a single line as illustrated in Figure 11. By contrast, the temperature characteristics of the glass are nonlinear and also not necessarily repeatable since 40 various characteristics of the glass are dependent on its prior history, as discussed above. Therefore, the key to making the process described above work is to select a glass and the temperature cycle for fusing the glass such that harmful 45 stresses are not developed during the process.

The process described above results in the glass having a contraction characteristic within the upper area of those that are possible. Using this temperature cycle discussed above, it is 50 practical to fuse thick (i.e. greater than 30

microns) glass layers directly to the outer surface of the diode body to form a layer protecting the PN junction. Using this cycle, it is also possible to incorporate the outer metal ring into the structure 55 with the outer metal ring maintaining the fused glass slightly in compression. Even though the process described here does not require outer metal compression rings as an essential part of the glass encapsulation, such components, if 60 desired for packaging or any other reason, can be incorporated into the process. Suitable metals for performing this function include Kovar, titanium and steel. Kovar is a trademark for an alloy consisting of 20% nickel, 17% cobalt, 0.2% 65 manganese with the balance iron. This alloy is also sold under the trade name Fernico. A suitable steel is type 304. This steel has a composition by weight of 0.08% carbon (max.,) 2.0% manganese (max.,) 1.0% silicon (max.), 19.0% chromium, 70 10% nickel with the remainder iron. Molybdenum has too low a temperature contraction coefficient resulting in the glass ring being placed in tension which is sufficiently high to rupture the glass. By contrast, nickel has a rather high temperature 75 expansion coefficient resulting in the glass being placed in sufficient compression to cause the rupture of the diode. It should be noted that outer rings of ceramic materials are also usable. For example, ceramics, including zircon (Z2Si04), 80 muilite (3AI2032Si02), porcelain, titanium (Ti02) and spinel (MgAI204) are also usable.

It should also be noted that thermal history for the glass tubing from which the glass rings are cut effects its thermal characteristics. Since 85 commercially available stress relieved glass tubing is made from molten glass all of the prefabricated glass rings cut from such tubing can be considered as having substantially the same prior thermal histories. However, it is believed that 90 slightly higher yields might be realized by carefully controlling the manufacturing process for these tubes. Additionally, the glass rings can be formed by sintering powdered glass. However, as previously mentioned, superior results have been 95 achieved by using rings cut from stress relieved glass tubing.

Claims

Claims

1. A semiconductor device comprising a semiconductor device body having opposed

100 surfaces with an edge therebetween; and a glass member disposed about the body und having an inner surface fused to said edge, said glass member having a first region of a lead borosilicate glass fused to said edge and a second region of a

105 zinc borosilicate glass disposed about said first region.

2. A semiconductor device according to claim

1, wherein said lead borosilicate glass is fused directly to said edge.

110 3. A semiconductor device according to claim

2, wherein said zinc borosilicate glass is disposed about and fused to said first region.

4. A semiconductor device according to claim 2 or 3 wherein said lead borosilicate glass has a

115 composition by weight of 26+4% Si02, 15+3% B203,45+3% PbO and 3 + 1% Al203.

5. A semiconductor device according to claim 2, 3 or 4 wherein said zinc borosilicate glass has a cvmposition by weight of 55±5% ZnO, 31 ±4%

120 B203, 8+2% Si02, 4.5+CeO and approximately 1.0% AI203.

6. A semiconductor device substantially as hereinbefore described with reference to and as illustrated in the drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained